1. Field of the Invention
The technical field relates to a method for manufacturing a light-emitting device having a thin film transistor, in particular, to a light-emitting device typified by an EL display device.
2. Description of the Related Art
In recent years, thin film transistors which include a thin semiconductor film with a thickness of several tens of nanometers to several hundreds of nanometers approximately and are formed over a substrate having an insulating surface, such as a glass substrate, have been attracting attention. Thin film transistors are widely applied to electronic devices such as integrated circuits (ICs) and electrooptic devices. In particular, prompt development of thin film transistors that are to be used as switching elements in image display devices typified by liquid crystal display devices, electroluminescent (EL) display devices, and the like is being pushed.
In an active matrix EL display device, a voltage is applied between one electrode and the other electrode, which sandwich an EL layer (including a light-emitting layer), of a light-emitting element provided in a pixel selected, whereby a current is generated in the EL layer and the light-emitting layer emits light. A viewer perceives this light emission as a display pattern. In an active matrix EL display device, pixels arranged in a matrix are driven with switching elements, thereby forming a display pattern on a screen.
The range of uses for the above active matrix EL display devices has been widening, and a larger screen, higher definition, a higher aperture ratio, and the like have been demanded. Further, higher reliability, higher productivity, lower production cost, and the like are required of active matrix EL display devices. An example of a method of enhancing productivity and reducing production cost involves a simplified process.
In an active matrix EL display device, thin film transistors are mainly used as switching elements. It is effective to reduce the number of photomasks used in photolithography in simplifying a process of manufacturing thin film transistors. For example, when one photomask is further used, steps of resist application, prebaking, light exposure, development, post-baking, and the like, steps of film formation and etching before and after the preceding steps, and further, steps of resist removal, cleaning, drying, and the like are required. Thus, even when only one photomask is additionally used in a manufacturing process, the number of steps increases significantly. As described above, a process is highly simplified or complicated depending on the number of photomasks; therefore, a number of techniques have been developed to reduce the number of photomasks in a manufacturing process.
Thin film transistors are broadly classified into top gate thin film transistors, in which a channel formation region is provided below a gate electrode, and bottom gate thin film transistors, in which a channel formation region is provided above a gate electrode. It is known that a bottom gate thin film transistor is more advantageous than a top gate thin film transistor in terms of the number of photomasks used. In general, a bottom gate thin film transistor can be manufactured using three photomasks.
With regard to conventional techniques for reducing the number of photomasks, complicated techniques such as backside light exposure, resist reflow, and lift-off are largely used, which often require a special apparatus. There have been concerns for decrease in a yield and the like due to a variety of problems caused by using such a complicated technique. Further, electric characteristics of thin film transistors have to be sacrificed in many cases.
As a typical means for reducing the number of photomasks in a process of manufacturing a thin film transistor, a technique of using a multi-tone photomask (called a “half-tone mask” or a “gray-tone mask”) is widely known. For example, Reference 1 (Japanese Published Patent Application No. 2003-179069) discloses a technique to reduce the number of manufacturing steps with the use of a multi-tone photomask.